1. Field of the Technology
The technology presented herein relates to a circularly polarizing plate and a liquid crystal display device. More particularly, it relates to a circularly polarizing plate which is suitable for a semitransparent vertically aligned (VA) liquid crystal display device or others, and a liquid crystal display device comprising the circularly polarizing plate.
2. Description of the Related Art
A circularly polarizing plate is widely used in a display device such as a liquid crystal display device as an optical member for converting incident light to circularly polarized light. In a liquid crystal display device used in a mobile device such as a cellular phone, a PDA (Personal Digital Assistant) and the like, high luminance, high resolution and low consumption power are necessary. As a display method satisfying these characteristics, a semitransparent display method which implements a display by switching between transparent display mode and reflection display mode or using those modes together has attracted attention. In the semitransparent liquid crystal display device using such semitransparent display method, use of the circularly polarizing plate enables both transparent display and reflection display. That is, in order to implement transparent display and reflection display at the same time in the liquid crystal display device, it is necessary to form a liquid crystal layer having a so-called multigap structure and to use the circularly polarizing plate comprising at least one polarizer (linearly polarizing plate) and one wavelength plate (retarder) as a polarization element. In this respect, performance of the circularly polarizing plate deeply affects display characteristics.
A previous circularly polarizing plate generally comprises a polarizer and a λ/4 retarder (which generates retardation of almost ¼ wavelength in light wave of a set wavelength). According to the previous circularly polarizing plate, in order to prevent discoloration of a gradation display and a black display in a liquid crystal display, it is necessary to control wavelength dispersion of the retardation, that is, to keep characteristics of a wavelength plate in different wavelengths, and to prevent reduction of display quality at the time of increase of a field of view (an elevation angle). On the other hand, as a technique to control the wavelength dispersion of the retardation, proposed is a technique in which a ¼ wavelength plate (λ/4 retarder) and a ½ wavelength plate (λ/2 retarder which generates retardation of almost ½ wavelength in light wave of a set wavelength) are bonded at a predetermined angle to provide a λ/4 retarder having a small wavelength dispersion of retardation (refer to Japanese Kokai Publication Hei-10-68816 (p. 1-5, FIG. 1), for example). According to this technique, wavelength dispersion of the retardation may be controlled by bonding the λ/2 retarder and the λ/4 retarder so that respective stretch axes form an appropriate angle. As a result, linearly polarized light may be circularly polarized light in a large wavelength band. In addition, a constitution of a semitransparent liquid crystal display device comprising the circularly polarizing plate having the above constitution is disclosed (refer to Japanese Kokai Publication 2000-35570 (p. 1-9, FIG. 1) and its correspondent U.S. Pat. No. 6,295,109 B1 (FIG. 17), for example).
However, in the above circularly polarizing plate having the constitution in which the λ/2 retarder is added, viewing angle dependence of retardation of the λ/2 retarder is great, which could reduce a field of view of the circularly polarizing plate. That is, in the stacked circularly polarizing plate comprising a plurality of retarders, since light incident from an oblique direction on the circularly polarizing plate is converted to elliptically polarized light which is largely different from perfect circularly polarized light, a viewing angle of a liquid crystal display device could be small. Therefore, in the circularly polarizing plate having the constitution in which the λ/2 retarder is added, there is room for improvement to reduce the viewing angle dependence.
To widen a viewing angle of a TN (twisted nematic) type liquid crystal display device, proposed is a technique which uses an optical compensation film with an inclined optical indicatrix (refer to “Technology of the GRP Formula for Wide-Viewing-Angle LCDs”, by Motohiro Yamahara and three other persons, Sharp Technical Journal, Sharp Corp., April 2003, No. 85, p. 19-23, for example). In addition, with respect to a technique which polarizes light incident from a light source through a circularly polarized light separating layer, a polarization element comprising at least one retarder and one circularly polarized light separating layer having predetermined characteristics is disclosed as means for improving light usage efficiency in the liquid crystal display devices (Japanese Kokai Publication 10-206636 (p. 1-10, FIGS. 1-5), for example). The circularly polarized light separating layer used herein has a function of passing right-handed circularly polarized light and reflecting left-handed circularly polarized light, for example. This polarization element is disposed between a polarizing plate and a backlight and used as a film for increasing a luminance in the liquid crystal display device. In this polarization element, the retarder prevents light incident from an oblique direction from being colored by the circularly polarized light separating layer.
However, there is room for improvement in these techniques so that the viewing angle characteristic is improved by converting light incident from the oblique direction to more perfect circularly polarized light in the circularly polarizing plate having a function of converting incident light to circularly polarized light.